Circadian rhythms are exhibited by all eukaryotic plants and animals, including man. Biological rhythms are periodic fluctuations in biological processes over time, including circadian as well as seasonal variations. Circadian, or approximately 24-hour, rhythms include the production of biological molecules such as hormones, the regulation of body temperature, and behavior such as wakefulness, alertness, sleep and periods of activity. Circadian rhythms are endogenous, self-sustained oscillations over 24-hour periods found in organisms ranging from prokaryotes to humans (J S Takahashi, et al. Science, 217,1104-1111 (1982)).
In nature, circadian rhythms are closely tied to environmental cues that impose a 24-hour pattern on many of these fluctuations. The regulation of circadian rhythms by signals from the environment involves "entrainment" of the circadian rhythm. The environmental signals which affect entrainment of the circadian rhythm are termed "zeitgebers", an example of which is the light-dark cycle.
The control of many circadian rhythms in mammals is mediated by the portion of the brain called the suprachiasmatic nuclei (SCN). In humans as well as other mammals, the circadian clock, which controls all endogenous circadian rhythms, is located in the SCN of the hypothalamus. Activity, alertness, core body temperature, and many hormones all have endogenous circadian rhythms controlled by the SCN. The SCN is the primary pacemaker for circadian rhythms in mammals. Circadian rhythms are primarily entrained by the light-dark cycle. One of the most important and reproducible characteristics of a circadian clock is that it can respond to exogenous light/dark signals. The circadian clock is composed of three parts: light-input pathways, a clock, and effector pathways. Light signals are conveyed by the retina to the SCN, and the pineal gland produces melatonin (N-acetyl-5-methoxytryptamine), which is regulated by the SCN. Information regarding light is conveyed from the retina to the SCN via the direct retinohypothalamic tract (RHT), as well as indirectly via the lateral geniculate nucleus (LGN).
It has been suggested in the art that excitatory amino acids are involved in the transduction of information regarding the light-dark cycle to the SCN. Acetylcholine, neuropeptide Y, GABA may play a role in the entrainment and/or generation of circadian rhythms in mammals. In addition, 5HT.sub.1 receptor functioning may play a role in modulating the phase of the SCN clock. Although the primary neurotransmitter of the retinohypothalamic tract is thought to be glutamate, substance P is also present in these fibers.
The neuropeptide receptors for substance P (neurokinin-1; NK-1) are widely distributed throughout the mammalian nervous system (especially brain and spinal ganglia), the circulatory system and peripheral tissues (especially the duodenum and jejunum) and are involved in regulating a number of diverse biological processes. This includes sensory perception of olfaction, vision, audition and pain, movement control, gastric motility, vasodilation, salivation, and micturition (B. Pernow, Pharmacol. Rev., 1983, 35, 85-141). The NK-1 and NK-2 receptor subtypes are implicated in synaptic transmission (Laneuville et al., Life Sci., 42, 1295-1305 (1988)).
Substance P is a naturally occurring undecapeptide belonging to the tachykinin family of peptides, the latter being so-named because of their prompt contractile action on extravascular smooth muscle tissue. The tachykinins are distinguished by a conserved carboxyl-terminal sequence. In addition to SP the known mammalian tachykinins include neurokinin A and neurokinin B. The current nomenclature designates the receptors for SP, neurokinin A, and neurokinin B as neurokinin-1, neurokinin-2, and neurokinin-3, respectively. More specifically, substance P is a neuropeptide that is produced in mammals and possesses a characteristic amino acid sequence.
Substance P is a pharmacologically-active neuropeptide that is produced in mammals and acts as a vasodilator, a depressant, stimulates salivation and produces increased capillary permeability. It is also capable of producing both analgesia and hyperalgesia in animals, depending on dose and pain responsiveness of the animal (see R. C. A. Frederickson et al., Science, 199, 1359 (1978); P. Oehme et al., Science, 208, 305 (1980)) and plays a role in sensory transmission and pain perception (T. M. Jessell, Advan. Biochem. Psychopharmacol. 28, 189 (1981)).
Substance P is present in neurons that are part of the circadian clock system, particularly in the SCN, and it may be involved in the transmission of photic information from the retina to the SCN. Substance P increases the firing activity of neurons in the SCN in vitro (Shibata et al, Brain Research, 597, 257-263 (1992); Shirakawa, et al, Brain Research, 642, 213-220 (1994)) and also increases the locomotor activity of rats (Treptow et al, Regulatory Peptides, 5, 343-351 (1983). The fibers of the retinohypothalamic tract are likely to synapse with substance P receptor-containing neurons, which have been identified in the rodent SCN (Takatsuji, et al. Brain Res., 698, 53-61 (1995)). In vitro, substance P has been shown to influence the firing activity and glucose utilization of SCN neurons, as well as to induce phase-shifts in the firing rhythms of SCN neurons (Shirakawa, et al. Brain Res., 642:213-20 (1994); Shibata, et al. Brain Res., 597, 257-63 (1992)). Furthermore, application of substance P to SCN neurons induces expression of the fos protein, which is only induced in response to photic stimulation (Abe, et al. Brain Res., 708,135-42 (1996)). Recent findings indicate that fibers immunoreactive for substance P are also present in the human SCN (Moore, et al. Brain Res., 659, 249-253 (1994); Mai et al. J. Comparative Neurol., 305, 508-25 (1991)). Thus, it is likely that in humans, substance P release from the retinohypothalamic tract is able to convey photic information from the retina to the SCN and influence circadian rhythms. Conversely, a substance P (or neurokinin-1) antagonist may be able to inhibit photic information from reaching the SCN, also influencing the circadian clock and circadian rhythms. The exact role of substance P with respect to circadian rhythms has not been previously determined.
The SCN and the circadian clock control the phases and rhythms of a number of hormonal rhythms in humans. One of the most well-characterized SCN outputs is to the pineal body, via a circuitous route from the hypothalamus to the spinal cord and then back to the pineal. The human pineal gland secretes melatonin in a circadian fashion, such that the plasma concentrations observed during the night are ten to forty times higher than those observed during the day. This plasma melatonin rhythm is a true circadian rhythm, and therefore not dependent upon the exogenous light-dark cycle, as it persists in blinded animals and blind humans. However, light is able to influence the endogenous melatonin rhythm. Light exposure during the night, when plasma melatonin concentrations are high, is able to rapidly suppress plasma melatonin to near daytime levels in a dose-dependent manner (C A Czeisler, et al. N. Eng. J. Med., 332, 6-11 (1995); McIntyre I M, et al. J Pineal Res, 6, 149-56 (1989); D B Boivin, et al. Nature, 379, 540-2 (1996)). The suppressive effects of light on plasma melatonin concentrations are believed to be mediated through the retina-SCN-pineal neural pathway (R Y Moore, et al. Science, 210, 1267-9 (1980)). Thus, because substance P, acting via NK1 receptors, communicates photic information from the retina to the SCN, a neurokinin-1 antagonist will be able to attenuate the effects of light on the SCN, thereby reducing the suppressive effects of light on plasma melatonin concentrations.
Circadian rhythms are also an important modulator of sleep. Although sleep is necessary for survival, its precise homeostatic contribution is unknown. Sleep is not a uniform state, but rather involves several stages characterized by changes in the individual's EEG. A non rapid eye movement (NREM) type (75 to 80% of total sleep time) ranges in depth through stages 1 to 4 (deepest level). Stage 1 sleep is drowsiness, in which the EEG displays a lower voltage, more mixed frequencies and deterioration of alpha rhythm relative to the EEG when the individual is awake. In stage 2, background activity similar to that of stage 1 is experienced, with bursts of slightly higher frequency "sleep spindles" and sporadic higher amplitude slow wave complexes. The third and fourth stages of sleep display increasing high amplitude slow wave activity. The separate sleep stage in which the individual undergoes rapid eye movement (REM) occupies the remainder of the sleep time and occurs 5 to 6 times during a normal nights sleep. REM sleep is characterized by a lower voltage, higher frequency EEG and other characteristics similar to those which occur when the individual is awake, whereas the other four sleep stages are categorized as NREM sleep.
Individuals vary widely in their requirements for sleep, which is influenced by a number of factors including their current emotional state. The natural aging process is associated with changes in a variety of circadian and diurnal rhythms. Age-related changes in the timing and structure of sleep are surprisingly common problems for older people, and are often associated with significant morbidity. With advancing age, the total amount of sleep tends to shorten. Stage 4 can decrease or disappear and sleep may become more fragmented and interrupted. Evaluation of sleep patterns in elderly people shows that the timing of sleep is also phase advanced, especially in women. This tendency to go to sleep and wake up earlier is very frustrating to older people who feel that they are out of step with the rest of the world. In addition, the quality of sleep in the elderly is diminished with a marked reduction in slow wave sleep, a reduction in the deep stages of sleep (especially stage 4), fragmentation of REM sleep and more frequent awakenings. Similarly, non-elderly people may exhibit disturbances in the normal sleep process. These changes in the structure of sleep have been correlated to more frequent napping, decreased daytime alertness and declining intellectual function and cognitive ability. Deprivation of REM sleep has been suggested to interfere with the memory consolidation involved in learning skills through repetitive activity, and slow wave sleep has been implicated as being important in consolidation of events into long term memory. Likewise, decreases in the length of REM stages of sleep may be associated with a decrease in cognitive function and learning, especially diminished retention of memory. Depression and insomnia may involve a disruption of normal circadian rhythmicity.
Sleep disorders generally involve disturbances of sleep, including circadian rhythm disturbances, that affect a subject's ability to fall and/or stay asleep, and involve sleeping too little, too much or resulting in abnormal behavior associated with sleep.
Numerous compounds are employed in the art to facilitate normal sleep and to treat sleep disorders and sleep disturbances, including e.g., sedatives, hypnotics, anxiolytics, antipsychotics, antianxiety agents, minor tranquilizers, melatonin agonists and antagonists, melatonergic agents, benzodiazepines, barbituates, 5HT-2 antagonists, and the like. Similarly, physical methods have been employed to treat patients with sleep disorders such as the use of light therapy or the application of modulated electrical signals to selected nerves or nerve bundles.
Nevertheless, the known threapeutic regimens suffer from numerous problems, including residual effects in daytime function, impairment of memory, potential for addiction, rebound insomnia, "REM rebound" which may be associated with increased dream intensity and the occurrence of nightmares, and the like. Accordingly, a more physiological way to enhance sleep, achieve a chronobiologic (circadian rhythm phase-shifting) effect or alleviate circadian rhythm sleep disorders would be highly desirable.